Manic Monday

Where do these guys come from who are selling like mad on Friday only to buy like mad on Monday?  It can't be the same people, can it?  Those taking on risk over the weekend have steadily collected a seemingly risk-free premium.  I will be shorting this gap up and expect some downward pressure in the first 30 minutes.  The rest of the day feels like a crapshoot, but I still have a slightly bearish bias.